Various types of apparatus are known for detecting gaseous impurities, such as halogens, in an ambient atmosphere such as air. Such apparatus rely on various discharge phenomena in the detection of impurities.
In one detection scheme disclosed in U.S. Pat. No. 3,742,475 to Liebermann et al., which is incorporated herein by reference, gaseous impurities are detected by the use of a pulsed corona discharge. A pair of electrodes disposed in an atmosphere under test are repeatedly pulsed with a voltage sufficient to cause a corona discharge in the continuous corona region and the average (d.c.) current component of such discharge is measured. The d.c. signal obtained in accordance with the U.S. Pat. No. 3,742,475 to Liebermann et al. is a highly sensitive indicator of the presence and concentration of gaseous impurities including substances which behave like gaseous impurities, such as air-borne liquids and particulate matter, all of which will be referred to herein by the phrase "gaseous impurities". Of special interest, the corona discharge current level diminishes with increasing concentration of halogen gases. As a result, the Liebermann et al. apparatus is capable of detecting halogen gases in low concentrations making it especially useful in detecting leaks from refrigeration systems utilizing freon and similar halogen-containing refrigerants.
Leak detectors in general must be calibrated to take account of initial conditions because it ordinarily is desirable to detect only significant changes in concentrations with respect to low background concentrations in the ambient atmosphere. Since background levels change with time, weather factors, location etc., and since the sensitivity of the apparatus is affected by use as well as environmental conditions, presently available detectors have to be recalibrated frequently. This is especially so when attempting to detect very low concentrations of an impurity. As will be apparent, manual calibration is time consuming, is frequently a bother, and can be a source of error if not performed carefully.